1. Field of the Invention
The present invention relates to a digital/analog (D/A) converter.
2. Description of the Related Art
In a prior art D/A converter (see: JP-A-57-83924), a digital-to-analog converting section includes a constant current source circuit having a plurality of binary-coding weight current output terminals and including a plurality of bipolar transistor type current switches driven by digital input signals. Each of the bipolar transistor type current switches is connected between one of the binary-coding weighted current terminals and an analog output current terminal. A reference voltage generating section generates at least two reference voltages and supplies them to the constant current source circuit. Particularly, one of the reference voltages is supplied to the bipolar transistor type current switches. A current-to-voltage converting section converts an analog output current flowing through the analog output current terminal into an analog output voltage in response to the analog output current and supplies the analog output voltage to an analog output voltage terminal. This will be explained later in detail.
In the above-described prior art D/A converter, however, since the current switches use bipolar transistors, collector currents flowing through the bipolar transistors, which contributes to the analog output current, are smaller than emitter currents flowing through the bipolar transistors. Therefore, in order to compensate for the caused error, an additional reference voltage source is provided in the reference voltage generating circuit, which makes the D/A converter more complex, thus increasing the manufacturing cost. Additionally, in order to operate the bipolar transistors of the current switches in a non-saturated state, a control circuit for adjusting the binary data signal is required, which also makes the D/A converter more comples. Thus, the manufacturing cost is further increased.
Also, in the above-described prior art D/A converter, since the analog output current of the D/A converting section is directly supplied to the current-to-voltage reference voltage generating section, the D/A converter is disadvantageous in terms of lowering the power supply voltage of the D/A converter.
It is an object of the present invention to provide a simplified D/A converter with a wide dynamic output range.
According to the present invention, in a D/A converter, a digital-to-analog converting section includes a constant current source circuit having a plurality of binary-coding weighted current output terminals and including a plurality of MOS transistor type current switches driven by digital input signals. Each of the MOS transistor type current switches is connected between one of the binary-coding weighted current terminals and an analog output current terminal. A reference voltage generating section generates at least one reference voltage and supplies it to the constant current source circuit. A current-to-voltage converting section converts an analog output current flowing through the analog output current terminal into an analog output voltage in response to the analog output current and supplies the analog output voltage to an analog output voltage terminal.
Also, a current mirror circuit is connected between the analog output current terminal and the current-to-voltage converting section.